Motor vehicle alignment systems are important for ensuring that the alignments of wheels on a vehicle are within the specifications provided by motor vehicle manufacturers. If the wheels are out of alignment, there may be excessive or uneven wear of the tires and vehicle suspension system. In addition, the performance of the vehicle, particularly handling and stability, may be adversely affected if the wheels are not properly aligned. As used herein, the term “wheel” or “vehicle wheel” refers to the tire and wheel assembly found on a motor vehicle. Such an assembly generally includes a conventional tire that is mounted on a metal wheel or “rim.”
The wheels of a motor vehicle may be aligned in a number of ways. For example, an operator or an alignment technician can use a vision imaging system such as a computer-aided, three-dimensional (3D) machine vision that employs optical sensing devices, such as cameras, to determine the positions of various objects. Examples of such apparatus and methods are disclosed in U.S. Pat. No. 5,724,743, entitled “Method and Apparatus for Determining the Alignment of Motor Vehicle Wheels” issued to Jackson, et al. on Mar. 10, 1998; U.S. Pat. No. 5,535,522, entitled “Method and Apparatus for Determining the Alignment of Motor Vehicle Wheels” issued to Jackson, et al. on Jul. 16, 1996; and Published U.S. patent application No. U.S. 2003/0065466A1, published Apr. 3, 2003, to Jackson, et al. and entitled “Self-Calibrating, Multi-Camera Machine Vision Measuring System”. All three of these patents and patent application are assigned to the assignee of the present invention and incorporated herein by reference. The Visualiner 3D3 Arago™ Aligner (Model # EEWA533A), for example, is a vision imaging alignment system that is available from the John Bean Company of Conway, Ark. (www.johnbean.com).
During an alignment process, a number of parameters are measured and adjusted to achieve proper wheel alignment. These parameters include camber, caster, steering axis inclination (SAI) and toe.
Camber is the angle of the wheel, measured in degrees, when viewed from the front of the vehicle. If the top of the wheel is leaning out from the center of the car, then the camber is positive, if the top of the wheel is leaning in, then the camber is negative. If the camber is out of adjustment, it will cause tire wear on one side of the tire's tread. If the camber is too far negative, for instance, then the tire will wear on the inside of the tread.
Caster is the angle of a steering pivot that the front wheels turn on, measured in degrees, when viewed from the side of the vehicle. If the top of the pivot is leaning toward the rear of the car, then the caster is positive, if the top of the pivot is leaning toward the front of the car, then the caster is negative. If the caster is out of adjustment, it can cause problems in straight line tracking. If the caster is different from side to side, the vehicle will pull to the side with the less positive caster. If the caster is equal but too negative, the steering will be light and the vehicle will wander and be difficult to keep in a straight line. If the caster is equal but too positive, the steering will be heavy and the steering wheel may kick when the vehicle hits a bump. Positive caster settings provide a greater degree of steering efforts, self-centering and stability to the vehicle. Therefore, nearly every car has a positive caster setting.
The toe measurement is the difference in the distance between the front of the tires and the back of the tires. It is measured in fractions or decimals of an inch in the US and is usually set close to zero which means that the wheels are parallel with each other. Toe-in means that the fronts of the tires are closer to each other than the rears. Toe-out is just the opposite. An incorrect toe-in will cause rapid tire wear to both tires equally.
SAI is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. This angle, when added to the camber to form the included angle causes the vehicle to lift slightly when you turn the wheel away from a straight ahead position. This action uses the weight of the vehicle to cause the steering wheel to return to the center when you let go of the steering wheel after making a turn. Because of this, if the SAI is different from side to side, the SAI will cause a pull at very slow speeds. Most alignment machines have a way to measure SAI; however it is not separately adjustable. The most likely cause for SAI being out is bent parts which must be replaced to correct the condition. SAI is also referred to as KPI (King Pin Inclination) on trucks and old cars with king pins instead of ball joints.
Camber, caster, and SAI are typically measured using inclinometers attached to the wheel. With camber, the inclinometer measures the angle that the plane of the wheel makes with the vertical. To measure caster, the wheel is turned through an arc, and the difference in the camber readings is used to derive the caster value. This procedure is called a caster swing.
SAI is measured in a manner similar to caster, except that the inclinometer used for the SAI reading measures the change in pitch angle of a line in the plane of the wheel as the wheel is turned through an arc. The SAI measuring inclinometer is aligned at 90° to the inclinometer used for reading camber and caster.
A typical alignment procedure includes first measuring the camber of the rear tires and the toe of the rear tires. Then the caster, the camber and the toe of the front tires are measured. Normally, this procedure produces accurate results. However, if the front wheels are not in near correct specification, then the camber and toe measurements of the rear tires will not be accurate. For example, if the front cradle is shifted and/or the front wheels are out of location, the adjustments made to the rear wheels will reference an inaccurate front center point.
Based on the foregoing, there is a clear need in this field for a new and improved method for measuring and adjusting the alignment of the wheels of a motor vehicle. In particular, what is needed is a new and improved method for measuring and adjusting the alignment of the wheels of a motor vehicle that takes into account that measurements and adjustments of the rear wheels of a vehicle can be effected by front wheels that are not in near correct specification.